1. Field of the Invention
The present invention relates to liquid crystal display using ferroelectric liquid crystal.
2. Description of the Related Art
Since liquid crystal display has features that it is thin and is low in power consumption and other features, the use thereof has been expanding in various articles from large-sized displays to portable information terminals and the development thereof has been actively made. Conventionally, for liquid crystal display, a TN system, an STN multiplex driving system, an active matrix driving system in which thin film transistors (hereinafter, it may be referred to as “TFT element”) are used in TN, and others have been developed and made practicable. However, nematic liquid crystal is used therein; therefore, the response speed of the liquid crystal material is as small as several milliseconds to several tens of milliseconds and it cannot be said that these sufficiently cope with display of moving images.
Ferroelectric liquid crystal (FLC) exhibits a very short response time in order of microseconds, and thus FLC is a liquid crystal suitable for high-speed devices. About ferroelectric liquid crystal, there is well known a bistable liquid crystal which has two stable states when no voltage is applied thereto and is suggested by Clark and Lagerwall (FIG. 4). However, the liquid crystal has a problem that the liquid crystal has memory property but graduation display cannot be attained since the switching thereof is limited to two states, namely, bright and dark states.
In recent years, attention has been paid to ferroelectric liquid crystal in which the liquid crystal layer thereof is stable in a single state (hereinafter referred to as “monostable”) when no voltage is applied thereto as a liquid crystal making it possible to attain graduation display by the matter that the director (the inclination of the molecule axis) of the liquid crystal is continuously changed by a change in applied voltage so as to analogue-modulate the light transmission thereof (NONAKA, T., LI, J., OGAWA, A., HORNUNG, B., SCHMIDT, W., WINGEN, R., and DUBAL, H., 1999, Liq. Cryst., 26, 1599., FIG. 4).
As the liquid crystal showing such mono-stability, in general, a ferroelectric liquid crystal having the phase change of cholesteric phase (Ch)-chiral smectic C (SmC*) phase without the transition to the smectic A (SmA) phase in the temperature lowering process is used.
On the other hand, as the ferroelectric liquid crystal, there is a material having the phase change of Ch-SmA-SmC* so as to show the SmC* phase via the SmA phase in the temperature lowering process. Among the ferroelectric liquid crystal material reported so far, most of them are those having such a phase sequence compared with the former material without the transition to the SmA phase. It is known that such a ferroelectric liquid crystal having such a phase sequence in general has two stable states with respect to one layer normal line so as to show the bi-stability (FIG. 5).
As a method for processing such ferroelectric liquid crystal for providing the mono-stability, a polymer stabilizing method can be presented. The polymer stabilizing method is a stabilization method by injecting a ferroelectric liquid crystal mixed with an ultraviolet curable monomer in a liquid crystal cell with the alignment process applied, and executing the ultraviolet ray irradiation in a state with the direct current or alternative current voltage applied for the polymerization. However, a problem is involved in that the production process is complicated and the driving voltage is made higher.
In general, as the technique for subjecting liquid crystal to alignment process, there is known a method of using an alignment film. The method is classified into the rubbing method and the photo alignment method. The rubbing method is a method of subjecting a substrate coated with a polyimide film to rubbing treatment to align chains of the polyimide polymer in the direction of the rubbing, thereby aligning liquid crystal molecules on the film. The rubbing method is excellent in controllability of the alignment of nematic liquid crystal, and is generally an industrially applicable technique. However, according to this method, there are problems of the static electricity or dust generation, unevenness of the alignment limiting force or the tilt angle due to the rubbing condition difference, irregularity at the time of the large area process, or the like. Compared with the nematic liquid crystal, due to the high molecule order, the alignment control is difficult, and thus it is not suitable for the alignment processing method for the ferroelectric liquid crystal, which can easily generate the alignment defect.
On the other hand, the photo alignment method is for aligning the liquid crystal molecule of the film by directing a light bean with the polarization control to a polymer or a monomolecale for generating the photo excitation reaction (decomposition, isomerization, dimerization) so as to provide the anisotropy to the polymer film or the monomolecular film. This method is advantageous in that the quantitative alignment process can be controlled without generation of the static electricity or the dusts, which is the problem of the rubbing method. However, even by using this method, an example of realizing the mono-stability operation mode using the ferroelectric liquid crystal essentially having the bi-stability has not been found so far. Therefore, in order to realize the highly precise color display by the color filter system or the field sequential color system by enabling the graduation display, due to the narrow material selection range for the ferroelectric liquid crystal, it has been difficult to achieve the various demanded characteristics.